1. Field of Invention
The present invention relates to a processing method and equipment for forming semiconductor devices. More particularly, the present invention relates to a photomask set for photolithographic operation.
2. Description of Related Art
In order to pack more semiconductors in a silicon chip, resolution in a photolithographic operation must be increased. According to Rayleigh""s theory, the resolution of a pattern after a light exposure can be represented by the formula:
R(resolution)=K1xc3x97xcex/NAxe2x80x83xe2x80x83(1) 
where xcex is the wavelength of the light source used in the light exposure, NA is the numerical aperture of the light-exposing instrument, and K1 is a process-related parameter whose value depends on the photoresist material, thickness of the photoresist layer and method of exposure. Therefore, aside from using a shorter wavelength light source, another means of increasing pattern resolution is to lower the value of K1.
However, as K1 is lowered to a value below 0.4, optimal exposure conditions are hard to attain if pitch-to-size ratio of pattern in the photoresist layer is too small or pitch-to-size ratio varies too much across a photoresist layer. This is because when the pitch-to-size ratio is too small, interference caused by light passing through a narrow slit is greatly intensified. Therefore, the pattern is often distorted. In addition, when the pitch-to-size ratio is not uniformly spread, a single photomask can hardly satisfy all the pitch-to-size ratios scattered across the pattern. Hence, a portion of the pattern may be distorted or some micro features are erroneously transferred. In other words, processing window of lithographic operation is greatly lowered.
Accordingly, one object of the present invention is to provide a photomask set and a photolithographic operation capable of increasing the process window of a pattern transfer operation.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a photomask set. Each photomask in the set has a different pattern formed thereon. To transfer a complete pattern to a photoresist layer, each of the photomask is used in sequence. In other words, multiple light exposures are conducted instead of a single light exposure. Pitch-to-size ratio of the pattern on each photomask is greater than the pitch-to-size ratio of the intended pattern in the photoresist layer. Pitch-to-size ratios of the pattern on each photomask are roughly identical but differ from the pitch-to-size ratio of the intended pattern on the photoresist layer. In addition, various types of patterns including linear-shaped blocks, rectangular islands or even contact openings can be formed on the photomask.
In this invention, multiple exposures with each light exposure using a different photomask are used to form a single pattern on a photoresist layer. By breaking up a pattern transfer to a photoresist layer into multiple steps, pitch-to-size ratio of the pattern on each photomask can be increased or made roughly identical to each other. Hence, the process window of each photolithographic exposure is increased leading to a leap in the overall process window of pattern transfer to a photoresist layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.